In the manufacture of hybrid integrated circuits, problems are encountered in bonding semiconductors to the circuit board or blank, in connecting the semiconductors electrically with the conductor tracks and in connecting other passive or active components and jumper wires to the board. In some applications the semiconductors may be attached by means of epoxy bonding, but the present invention is not concerned with applications for which this technique can be used. It is concerned only with all-metal assembly systems such as are required for meeting difficult environmental conditions, particularly as required by certain military specifications.
A typical integrated circuit assembly made according to the prior art is shown in section in FIG. 1. On a substrate of aluminum oxide, beryllium oxide or sapphire, a layer of 80-20% nickel-chromium is sputtered or vacuum-deposited to a thickness sufficient to obtain the required sheet resistance (typically 200-300 A). A layer of pure nickel approximately 1000 Angstrom units thick is then deposited over the nickel-chromium layer, and this is followed by an electro-deposition of pure gold 30 to 150 microinches thick.
Attachment of components, etc. to the board described above requires high-temperature eutectic bonding (375.degree. C, typically) for attaching semiconductors to the substrate and moderate-temperature solder (300.degree. C) for attaching passive components to the substrate. Low-temperature solder is used for mounting the substrate into a package and also for sealing the package. Since semiconductors are susceptible to damage from excessive heat, any reworking or repairs requiring moderate to high temperatures carry substantial risk of further damage or destruction of the integrated curcuit. In actuality, it has often proven impractical to attempt to repair such devices, and they have had to be replaced.
Another difficult problem occurs in effecting solder connections to the gold surface without risk of immediate or long-term failures. Low-temperature solders are not compatible with gold because they normally contain tin which absorbs the gold, thereby disrupting electrical conduction through the associated conductor. High-temperature solders are of high lead or gold alloys and are difficult to use in some applications because they can cause changes in the resistance of certain thin film devices. Because the gold alloys are "hard" solders, stress relief in multiterminal components is not possible, causing potential physical failure of such components. Also, the heat required is prohibitive to certain discrete components.
Where ultrasonic wire bonding techniques are used, it is necessary to have a surface which is compatible with the aluminum wires used on most semiconductor components. Bonding aluminum to gold frequently results in the formation of intermetallics (sometimes called "purple plague") which adversely affects the strength of the bond. These intermetallics, combined with a bulk diffusion process (Kirkendall voiding) is often destructive to the bond, reducing the circuit reliability.
The nickel-chromium layer is deposited for the purpose of supplying a resistive layer, and after the resistor patterns are formed the circuit board assembly is normally heat-treated to allow the resistors to increase in resistance value through oxidation. This heat-treating process normally requires several hours before a reasonably stable value is reached, which is time-consuming and expensive and still leaves much to be desired in terms of long-term stability of resistance values. Also, the times for heat-treating vary according to the characteristics of each particular substrate. Alternatively, the resistors are coated with a protective layer such as silicon monoxide which protects the resistors from oxidation so that the resistance stabilization is largely brought about through annealing. This latter system often involves depositing patterns through masks--an inefficient process.
A similar problem has been experienced in attempting to bond semiconductor chips, etc. to the integrated circuit boards. The semiconductors may be supplied without gold or with an evaporated or an alloyed gold layer for making electrical contact between the mounting surfaces of the semiconductor and the conductors on the circuit boards. Because of the above described problems in soldering to gold, low-temperature tin bearing solders cannot be used on those semiconductors. Techniques used involve either the use of conductive epoxy, which has disadvantages in poor resistance to high temperatures, susceptibility to resistance changes, and unreliability as to electrical connections, or high-temperature eutectic brazing alloys which generally render the connections unrepairable and which require that semiconductor dice be mounted individually, resulting in excessive exposure of the assembly to elevated temperatures.
With the requirements described above, it will be apparent that a relatively high skill level is required of those doing the various tasks required to manufacture such integrated curcuits, and any significant reduction in the skill level required of operators will result in appreciable savings in the cost of producing such circuit boards.